The present invention relates to a light spot position control system and method in which a position control such as a focusing control, a tracking control and/or a jitter correction of a light spot focused onto a recording medium is executed by servo signals (or error signals) derived as sampled signals from servo areas intermittently provided in the recording medium, and more particularly to such a system and method suitable for use in an optical information recording and reproducing apparatus such as an optical disk apparatus or an optical tape apparatus.
In an optical disk memory apparatus, a position control of a light spot such as a focusing control, a tracking control and/or a jitter correction is necessary in order to correctly record and reproduce data. In an optical disk apparatus put into practical use at the present time, a continuous control method has been employed for such a position control. The continuous control method is one in which for example, in the case of the tracking control, an error signal derived from a guide groove or pre-groove provided on the disk along the direction of rotation of the disk (or the direction of a track) is always detected and this error signal is continuously supplied to a control system to perform a desired control.
Recently, on the other hand, attention is directed to a method in which a control by sample values is carried out for performing the tracking control and/or the focusing control. A sampling control in an optical disk apparatus is disclosed by "Optical Mass Data Storage", SPIE Vol 529, pp. 85-88 and 140-144 (1985), U.S. Pat. No. 4,364,118 and U.S. Pat. No. 4,443,870.
In many cases of the sampling control in the optical disk apparatus, 1000 to 3000 servo areas (sample points) for obtaining servo signals for the focusing or tracking control are intermittently provided in one track round of the disk, data areas are provided between the servo areas, and the length of each servo area is not larger than 10% of that of each data area. Great merits of the use of the sampling value control in the optical disk apparatus are that since the data and servo areas are separated in terms of time and space, there is no fear that a servo system is disturbed by a modulated light of strong intensity during the recording of data and that the servo system encounters no influence of gain variation due to a change of reflectivity of the disk before and after the recording of data pit, etc.
A problem in the case of carrying out the sample value control is a measure to counter the case where some of the servo areas are destroyed by defects or the like of the disk and hence a correct error signal cannot be sampled or detected from the servo area. In the case of the continuous control, since an error signal is continuously obtained, for example, the influence of minute defects existing in the guide grooves can be reduced to a certain extent by averaging the error signal by means of a low pass filter or the like. On the other hand, in the case of the sample value control, since each sample point represents an error signal of servo areas before and after that sample point, the presence of an error in an error signal obtained from the sample point is equivalent to the presence of an error in the servo areas before and after that sample point. In the sample value control, it is general that the number of sample points is by nature selected to a minimum within a range in which the control system stably operates. Therefore, to eliminate the above-mentioned error in a sample value by a low pass filter is difficult as compared with the case of the continuous control. Accordingly, the sample value control has a problem that the influence of erroneous samples is liable to become large as compared with the continuous control.
As a countermeasure of the erroneous samples, the present inventors have proposed a tracking control in which reproduction signals from pits wobbled in the same direction between two adjacent servo areas are differentially detected to determine the normal or abnormal of the wobble pits in accordance with the normal or abnormal of the detected differential output and the use of a tracking signal detected is stopped immediately upon detection of the abnormal wobble pits so that a correct tracking signal detected at a one-preceding sample point is used (see U.S. Ser. No. 71,347 filed on Jul. 9, 1987 in the name of Wasao Takasugi et al). The proposed tracking control is characterized in that the abnormal of each wobble pit itself is monitored rather than the detection of the abnormal of a tracking signal detected from pits wobbled right and left.
The present inventors have further proposed a light spot position control system in which whether or not the latest sample value detected from servo areas intermittently provided falls within an allowable deviation range is checked, and if it is out of the allowable deviation range, the sample value decided to be normal or correct at a one-preceding sample point is used as it is or a prediction value obtained by use of a curve derived from a plurality of previous sample values by a least square method or a prediction value obtained from the previous sample values by a linear prediction method is used (see U.S. Ser. No. 72,095 filed on Jul. 10, 1987 in the name of Masatoshi Ohtake et al., now U.S. Pat. No. 4,785,442).